1. Field of the Invention
This invention relates to an optical information processor and more particularly, to an apparatus for tracking an information track by means of diffraction light from the information track.
2. Description of the Prior Art
In a conventional optical disk for storing digital information in such a manner that the information can be optically recorded and reproduced, an additional recording system has been proposed in which a groove for guiding a light beam is defined in advance on the optical disk in order to additionally record the information, and when the information is recorded, a light beam spot is guided by this guide groove to sequentially record the information (Press Information Philips, Nov. 7th, 1978).
On the other hand, an apparatus has also been proposed in which the diffraction light from an information track on an optical disk is received by two light detectors that are juxtaposed with each other to interpose the track between them and the output of each detector is differentially operated so as to track the guide groove (e.g., Japanese Patent Laid-Open No. 60,702/1974). However, since the optical detectors are disposed in the far-field zone of the diffraction light, offset is liable to occur in a tracking signal if the optical beam is moved or the disk is tilted so that tracking can not be made normally.
FIG. 1 illustrates schematically why offset occurs in the tracking signal due to the spot movement on the surface of the optical detector in a tracking signal detection apparatus using the diffraction light. FIGS. 1A(a) through 1A(d) are plan views of the positions of the light spots on the optical detector (having the light receiving surfaces 1, 2), and FIGS. 1B(a) through 1B(d) are sectional views corresponding to the center line A--A', respectively, and illustrate the intensity distribution of the light spot. FIG. 1(c) is a sectional view showing the relation of the intensity distribution of the light spot converged by an objective lens on the disk surface and the position of the groove. In the case of FIG. 1(C)(a), the center 7 of the light spot is placed on the position of the disk surface where the groove does not exist and in the case of FIG. 1(C)(b), the center is in agreement with the center 8 of the groove. In the case of FIG. 1(C)(c), the light spot is located at the extreme left of the groove (or inside the rotation of the disk) and in the case of FIG. 1(C)(d), the spot is located at the extreme right (or outside the rotation of the disk). In FIG. 1, the spot 4 on the optical detector, represented by dotted line, is generated by the inclination of the optical axis of the objective lens or when the optical axis is moved in order to follow up the off-center.
In FIGS. 1A(a) through 1A(d), the light receiving surface of the optical detector is divided into two portions 1 and 2 and the gap between them is extremely small. When the spot on the disk surface is situated at the center of the groove 6, the spot on the optical detector assumes the position shown in FIG. 1B(b) and the same quantity of diffraction light is incident to the two light receiving surfaces 1 and 2, as represented by solid line 3. If the position of the spot moves under this state due to the inclination of the disk, however, the state such as represented by dotted line 4 is established.
The tracking signal obtained by the diffraction light generally uses the differential signal of the outputs of the light receiving portions 1 and 2 in order to detect the unbalance of the diffraction light. If the spot position described above moves, the differential signal between the outputs of the light receiving portions 1 and 2 does not become zero, even though the center of the spot 7 on the disk surface is in agreement with the center 8 of the groove 6. Thus, offset occurs in the tracking signal.
Furthermore, in the case of FIG. 1(C)(c) where the center 7 of the spot on the disk surface is situated at the extreme left of the groove 6, the intensity distribution of the spot on the surface of the optical detector becomes such as shown in FIG. 1(B)(c), so that the quantity of light incident to the light receiving portion 1 becomes great as shown in FIG. 1A(c), the unbalance takes place between the differential outputs of the light receiving portions 1 and 2 and hence, the quantity and direction of deviation between the center of spot on the disk surface and the center of the track can be detected. If the center of the spot on the disk surface is situated at the extreme right of the groove as shown in FIG. 1(C)(d), the intensity distribution of the spot on the surface of the optical detector becomes such as shown in FIG. 1(B)(d), and the quantity of light incident to the light receiving portion 2 becomes great, as shown in FIG. 1A(d), so that the differential outputs of the light receiving portions 1 and 2 become opposite to those shown in FIG. 1(C)(c).